1. Field of the Invention
The present invention relates to a bar code reading device and a bar code reading method using an image pickup element (image sensor).
2. Discussion of the Prior Art
In the bar code reading device using an image pickup element, the entire range of a bar code to be read out from one end thereof to the other end are simultaneously illuminated with light projected from an LED (light emitting diode) array. A CCD (charge coupled device) image sensor, for example, simultaneously receives light reflected from the bar code, and transforms it into electrical signals. The electrical signals are converted into numerical values, and the converted numerical values are analyzed, whereby bar code information is read out. Such a reading operation is repeated till a predetermined measurement end condition is satisfied. One reading operation will be referred to as one scan.
The processing speed becomes higher as the frequency of the reading operation (the number of scans per unit time) is higher. A time that a bar code is imaged on the image pickup element is inversely proportional to the number of scans per unit time. Therefore, as the number of scans per unit time increases, an amount of received light per scan becomes smaller, and hence a read rate (a ratio of a number of successful readings to a number of reading operations) becomes smaller. The number of scans per unit time is set at a fixed value in advance by a hardware construction of each bar code reading device.
Generally, in the bar code reading device using the image pickup element, the entire range of the bar code is simultaneously illuminated with light, and the light reflected from the bar code is received for a preset time. Accordingly, if the bar code is moved during the measurement, it is impossible to accurately read out bar code information. In other words, to accurately read out bar code information by using the bar code reading device, it is necessary to put an object with a bar code label in a standstill state.
In production lines in a factory, there is a strong demand to accurately read a bar code attached to a moving object by using a small bar code reading device provided with the image pickup element. To meet the demand, there is proposed a method of measuring a moving bar code in a manner that the LED is lit on for a time duration shorter than a time that the bar code is imaged on the image sensor.
In this case, as the lighting time of the LED is shorter, the adverse affect by the movement of the bar code is more lessened. However, the read rate is decreased since the amount of light projected on the bar code is reduced. It is for this reason that the lighting time of the LED is preset to a proper fixed value by a hardware construction for each bar code reading device designed for its use purpose.
The best number of scans and the best lighting time of the LED are determined depending on use conditions, such as presence or absence of a bar code to be measured, a moving speed of the bar code, measurement distance, and the like. For this reason, it is desired that in reading bar code information, a user may set the bar code reading device in the best operating state in accordance with current use conditions.